Since a liquid crystal display has such various advantages as is operable with a low voltage and low power consumption, and capable of miniaturization and thickness reduction, development is proceeding for such applications as a monitor of a personal computer and portable devices, and a TV. Along with this, recently, development of a large size screen proceeds.
As well as a large size screen, in a liquid display device, thickness reduction of a whole image display device also proceeds. The thickness reduction of an image display device can be achieved by constituting an image display device using a panel provided with a substrate made of thin glass or resin. However, when such liquid crystal panel displays black screen after having been exposed to a high temperature and high humidity, there occurs a “circular unevenness” phenomenon, in which light leaks circularly, on a panel (screen), which brings about serious problem on the display performance.
On this occasion, observation of a panel teaches occurrence of warpage in the panel. That is, when the image display device is observed from the front side (viewer side), the central portion of the panel is concaved and the edge portions warp toward the front side.
The occurrence of aforementioned warpage of a panel is caused by the fact that, relative to a substrate made of glass or resin which does not generate warpage in nature, various types of members laminated on the viewer side and the back side thereof (hereinafter, the back side relative to the viewer side is occasionally referred to as “the back side” simply) generate expansion/contraction due to heating and moisture absorption/desorption to result in difference in the expansion/contraction between the viewer side and back side, thereby disrupting a balance of forces between front and back sides of an image display device. In an ordinary image display device, the viewer side surface of the panel is opened, but the rear face is mounted in a casing to become in a quasi sealed state. It is considered that, consequently, difference occurs in heating and moisture absorption/desorption amount between the front side laminated body and the back side laminated body between which the substrate is interposed to result in occurrence of difference in expansion/contraction, too, and generation of warpage.
Taking a liquid crystal display device as an example, a liquid crystal display device is manufactured by arranging polarizing plates for producing polarized light on both sides of a liquid crystal cell constituted of liquid crystal sealed between glass substrates, laminating various optical elements such as a retardation plate, an antireflection film or a brightness-enhancing membrane according to need, fixing the periphery thereof with a fixing frame comprising a metallic plate such as stainless steel plate, which is called a “bezel,” to form a liquid crystal module, and assembling and housing the liquid crystal module with other constitutional members in a casing.
According to such reason that, temperature rises due to a backlight when a light source switch of a liquid crystal display device is on, sometimes difference in temperature or humidity may occur between the front side (viewer side) and the backlight side of the device. In this case, it is considered that the front side laminated body and backlight side laminated body including a polarizing plate are exposed to different temperature or humidity conditions while taking the liquid cell as a boundary, and that respective laminated bodies are subjected to the influence of the difference in these conditions. When warpage occurs in a panel, the fringe portion or four corners of the panel not only contact with the casing, but also stick fast to the backlight arranged on the back side to generate display performance problems. Further, a “corner unevenness” phenomenon in which light leaks unevenly from four corners of a panel (screen) when the screen is in black level of display, which sometimes causes a very large problem in display performance.
In order to improve warpage of a panel due to environmental alteration, in JP-A-2003-149634, in a liquid crystal display device prepared by arranging polarizing plates comprising a polarizer with a protective film on both sides of a liquid crystal cell and further laminating a brightness-enhancing film to the back side polarizing plate, the thickness of the protective film used for the front side polarizing plate and the thickness of the protective film used for the back side polarizing plate are set to be not equal. However, when the protective film of the laminated body on the viewer side arranged on the front side was thinned, there was such problem that the polarizer tends to deteriorate easily due to humidity to lower optical performances.